Shielded condenser microphone



Dec- 18, 1951 P. s. vl-:NEKLAsl-:N

SHIELDED CONDENSER' MICROPHONE Filed Feb; 24, 1950 INVENTOR. 1.7304 .5I TNE/LASEM B 1 m/#Ma @Q1/a( I;- L64 V156 ry-OeA/Eys. u

Patented Dec. 18, a 1951 SHIELDED CONDENSER MICROPHONE Paul S. Veneklasen, Monrovia, Calif., assignor to Altec lLansing Corporation, Los Angeles, Calif., a corporation of Delaware Application February 24, 1950, Serial No. 146,131

4 Claims.

This invention is concerned generally with condenser microphones, and, more particularly with improved mounting means for the xed plate of the microphone condenser.

In order to provide adequate sensitivity and suitable damping of the microphone, the iiXed condenser plate must be spaced very close to the vibratory diaphragm which forms the movable condenser plate. Such close spacing can be uniformly maintained only if the xed plate is very rigidly mounted within the microphone housing. On the other hand, the two plates of the condenser must be very well insulated from each other to avoid leakage of the relatively high polarizing voltage across the insulating material, since such leakage causes noise andy other defects in the microphone output.

lIt is difcult, if not impossible to nd dielectric materials which combine the'properties of very high and uniform insulation andr eXtreme mechanical strength and Stability. In previous condenser microphone structures it has been necessary to compromise between maximum mechanical and maximum electrical stability of the supporting means for the fixed condenser plate.

An important object of the present invention is to provide an insulative mounting for the xed plate which is so shielded electrically as to overcome the difculties that normally result from the use of materials having only moderate insulative qualities. By the provision of such shielding, it becomes feasible to select the insulating material of the mounting primarily for itsy mechanical properties.A` The mechanical rigidity and stabilityv is thereby greatly increased, permitting closer tolerances to be maintained throughout the construction of the microphone, and, in particular, permitting a closer and more stable Working spacing between the fixed and vibratory plates of the microphone condenser. That improves the sound response of the microphone, and permits the physical size of the apparatus to be reduced while maintaining an effective electrical signal.

A full understanding of the invention and of its further objects and advantage will be had from the following description of a particular preferred manner of carrying it out. Details of that description and of the appended drawings, which form a part of it, are for purposes of illustration only, and are not limitative of the scope of the invention.

In lthe drawings:

Fig. 1 is an elevation of a typicatpreferred condenser microphone and preamplier structure, embodying the invention;

Fig. 2 is a fragmentary vertical axial section, taken on line 2 2 of Fig. 1, and showing the microphone proper and connections at enlarged scale;

Fig. 3 is a fragmentary section taken on line 3 3 of Fig. 1, at an intermediate scale, showing a preferred manner of supporting the preampliiier tube; and

Fig. 4 is a schematic diagram showing typical electrical circuitary adapted for carrying out the invention.

In the preferred embodiment illustrated, the microphone proper is designated by the numeral l0, and is removably mounted at one end of the tubular support I2, which acts also as housing for the preamplifier tube I4 and its electrical connections.

The microphone proper I0 comprises the generally cylindrical case 2l), within which are mounted the vibratory diaphragm 22 -and the closely spaced back plate 24, constituting respectively the movable and the xed plates of the microphone condenser. Diaphragm 22, which is of, or coated with, conductive material, is peripherally mounted between a fixed annular shoulder of the mounting ring 26 and the pressure distributing ring 28, resiliently urged toward the diaphragm by the annular spring 3U. Mounting ring 26 is threaded into internal threads in housing 20, being spaced by the spacing ring 3| from the annular shoulder 33 of the housing, which serves also as a seat for spring 3l). This mounting also serves to electrically connect the diaphragm to the case.

Back plate 24 is xedly mounted in coaxial relation to the ring 32, and is insulated therefrom by outer and inner annular insulating rings or bushings 34 and 36. Those two rings of insulating material are separated by the shield ring or sleeve 38, formed of conductive material. Typical electrical connections for shield ring 38 for carrying out the present invention are described below.

The parts just mentioned comprise the back plate assembly, which is preferably assembled by molding the insulative material of rings 313 and 36 directly in position between the metal parts 24, 32 and 38, the surfaces of the latter preferably being roughened, as by knurling, at the areas of contact to improve the effective strength of the bond. That back plate assembly, designated generally by the numeral 40, is mounted securely in mounting ring 26 by threading ring 32 directly into the depending skirt portion of the mounting ring. Such threaded assembly permits convenient adjustment of the -spacing between diaphragm 22 and the adjacent face of back plate 24. After that adjustment has been made, the position of ring 32 with respect to mounting ring 26 is locked by tightening lock ring 42, thereby pressing clamp ring 44 upward about the skirt of ring 2G, compressing the internal threads of the latter. The physical arrangement and relationship of parts described above, apart from shield ring 38,are further described and claimed in the copending patent application of William J. Moreland, Jr., entitled Condenser Microphone Structure? Ser. No.

146,133, filed February 24, 1950, now Patent No.-

2,567,889, granted September. 11, 195.1.

A generally iiat end wallk .''on microphone case 29, preferably formed integrally with the cylindrical side walls of the case, forms a sound chamber 52 directly above diaphragm 22. (Such terms as above,"top,. and the like are used here for clarity of description with reference to the drawings, and not as limitations upon the orientation of the elements described.) Sound passages are provided, as at 54, by means of which the pressure within chamber 52 varies in accordance with the periodically varying sound pressure outside the housing. Passages 54, as illustrated, comprise transverse slots out just above annular shoulder 33 and opening, through the internal face of end wall D near its periphery. That passage structure and its relation to other parts of the microphone, as typified in the4 present embodiment of the present invention, are more fully described and claimed in the copendingpatent application of George E. Carrington, entitled .Microphone Sound Passage Structure, Serial No. 146,075, filed February 24 1959, now Patent No. 2,567,875, granted September 11, 1951.

The figures'illustrate a preferred manner of mounting and providing electrical connections to the microphone proper. The support I2 comprises a lower hollow body portion 60, an intermediate, relatively narrow hollow neck portion 62 and a hollow upper or head portion 64l which includes a microphone receiving socket. The latter, as illustrated, employs internal threads which mate with external threads on the cylindrical wall of microphone case 20, such threadsv typifying any suitable releasable securing means for the microphone. The external form of the assembled microphone I0 and support I2 is claimed as an ornamental design in the copending dev sign patent application of Vernon K. AlbertSer.

Body portion 6,6) of the microphone supportV forms a generally cylindrical housing in which preamplifier tube I4 is mounted. The lower end of that housing is closed by the male portion of an electrical connector, indicated at 6B.v The female portion 'IG of the connector is mounted on a base T2, from which an electrical cable 'I4 makes connection to any elements of the electrical system that are not mounted within support I2. Connector portions 68 and 'Ill are releasably secured together in any conventional manner, forming a mechanical as well as electrical conf nection between base 'I2 and support l2, so that, for example, the entire assembly may be suspended from cable 'I4 when in use.

In the embodimentillustrated, connector portion` 68 is mounted, as by the screws 16, in a thinwalledsleeve 18 which extends both above and l below the body of the connector, the lower portion forming a protective skirt about the connector pins and providing also a secure mechanical connection with connector portion I by slipping over it. A bracket formation, typically comprising two independent arms 82, supports a conventional tube socket 9d, adapted to mount preamplifier tube I4. If, as in the present embodiment, that tube is of the type having its grid contact in the base, the socket and tube are mounted on brackets 82 in inverted position, as illustrated. Connections are then made from tabs A'I9 of connector 68 to corresponding tabs 9| of` tubev socket 90 by means of sections of insulated wire 92. Those wires, only one of which is shown in full iriFg.V 3, are preferably resiliently tied tothe body of tube I4, as by a iiexible ring 94.

As will be explained, the electrical connections to theY microphone proper in accordance with the invention are of three types. As in previous condenser microphones., theA microphone condenser is connected across the inputof the amplifying system, the microphone case 20 and conductive diaphragm 22 preferably being connected to ground; and back plate 24, which isinsulated from the case, being `connected to the control grid of the preamplier tube.v The third connection is to shield ring 38,.l In the.v preferred embodiment illustrated, the preamplifier tube is a pentode used as a cathode follower, and shield ring 38 is connected directly to the suppressor grid of the tube, and is related to the tube; cathode as will be described.

The threev types of .connection justv mentioned are provided in the-,present embodiment in the following preferred manner. Ground connection from microphone.y case `2Il is carried Vthrough its mounting connection to support sleeve I2, which is a metal, tube of unitary structure that iits releasably over sleeve 18. Electrical contact to the grounded pin of connector 68 may be made via sleeve I8 and bracket 82 and by a short soldered lead, indicated at85.

Connection to shield ring 38 `is made Via a shield sleeve Iwhich Aismounted coaxially with'- in support I2 in spaced and insulated relation thereto Asillustrated, the lower end of sleeve Illu rests upon the upturned central tongue 96 of inverted tube socketl Ell, and is therebyl supported and transversely located. Sleeve Hill is electri cally connected to the appropriate one of tube socket tabs SI., as Vby a Wire 9.1, soldered at both ends. The; upper end of; sleeve H30 is held in coaX-ially spaced relation within the cylindrical inner face H12` of; vsupport I2 by the insulative bushing,` IEM,v which is4 preferably fixed to, sleeve; I DI! and freely slidable within support I 2.V A teler: scopingy Contact,` Sleeve IBS; is slidably mounted on thel upper end of sleeve H10, and isyieldingly` urged upwardasby-thecoilspring |08,l which also affordssoldered` electrical contact between the; relativelymovable sleeves. The upper spring tip Ill] is bent radially inward, and. extends. through the; relatively-large hole |12 in sleeve Ill, thereby forming a limit stop` which limits the relative telescopic movement of the tubes 00, HlI. The upper: end of contact sleeve |05, directly engages the lowerface of guard ring 3.8;, on which it is; centered by radial flange I I 4to insure good contact.

Connection to back plate 24 of the microphone condenser,` is, made Via the conductor |20, which is; axially centered within sleeye; mi) by the4 relativelythick layer of iiexible insulation 1.22', conductor and, insulation-- preferably` forming` a. unitary cable. That cable `is bent radially outward through an aperture |24 at the lower end of sleeve |00, making conductor |20 readily available for soldering directly. to the appropriate one of tube socket tabs 9|. At its upperend cable |20, I 22 leads into an insulating bushing |30 within sleeve |00. Bushing |30 is bored and counterbored to receive an axially. slidableheaded contact pin |32, .which is urged upwardly by coil spring |34, preferably soldered directly to the pin and to the upper end of conductor |26.into rm contact with the lower exposedface of back plate 24.

The preferred structure described has the advantage of easy disassembly. By removal of screws 33, support sleeve I2 is released for axial movement with respect to the internal structure, and may be completely removed, together` with microphone l0, by simply sliding it upward. During that process, connector 68 is preferably lleft attached to connector To on base 12. Such removal of housing |2, and microphone l0 breaks the spring-pressed contacts between guard ring 38 and sleeve |86, and between back plate24 and pin |32, leaving connector. 68, bracket 82, tube socket 80 with its tube |4, and the connecting sleeve and cable |20, |22 supported on base 12 .and fully accessible for inspection and repair. Alternatively, the microphone proper may be removedV from its support without disturbing any of the remainder of the apparatus, simply by unscrewing it from support l2.V The electrical spring contacts are again opened at the points just referred to. The particular connection structure herein shown and above described is for illustrative purposes only andy is not herein claimed in and of itself.

Fig. 4 shows in schematic form typical electrical connections for carrying out the invention. The plate |50 of preamplifier tube i4 is bypassed to ground via condenser |14, and is connected via resistor |52 tothe positive terminal of a suitable source of direct current, indicated typically by battery |54.- The negative terminal of batteryA |54 is connected to relative ground at |56 via line |55. The case 20 and support |2 of microphone I0 are connected to ground via line |51, grounding vibratory microphone diaphragm 22. Tube cathode |60 is connected to ground through the two resistors |62, |764, which comprise the cathode load and form a voltage divider. Suitable voltage for tube screen 68 is provided from the positive side of battery |54, as via voltage droppingI resistor |12, with bypass condenser to the cathode as indicated at |'|6.

The fixed back plate 24 of the microphone is connected via contact pin |32 and line |18 directly to tube control grid |80, the usual grid resistor being preferably omitted, so that the potential of the control grid and back plate system is determined by the balance of positive and negative grid current. In operation, that system comes to a stable voltage that is more negative than the cathode by a suitable bias voltage. The ratio of cathode load resistors |62, |64 is so chosen that the drop across |62 equals that bias voltage. The junction of |62 and |64 is thus at the same average potential as the control grid and back plate system, and the voltage standing across the plates 22, 24 of the microphone condenser is substantially the voltage drop in cathode resistor |64.

In accordance with the present invention, shield ring 38 of the microphone is so connected that its potential follows that of the microphone back plate closely, both as to the xed equilibrium componenta'nd'as' to the variable component that results from vibratory movement of microphone diaphragm 22; while at the same time the shield ring has an impedance'to ground that is relatively v-verysmall in comparison to that of the microphone back plate, the latter impedance being substantially the tube grid resistance in the present preferred embodiment, and being typically many megohms. even in circuits employing `a grid resistor. That type of connection is preferably accomplished, as illustrated, by connectingthe shield ring directly via line to the junctionbetweenvoltage dividing cathode resistors i 62 and |64, selected as described, with a large bypass condenser to the vcathode itself, as at |82. Tube screen grid |84 is preferably tied directly to the shield ring and its described-connections, but may alternatively be connected in any conventionalmanner, as directly tothe tube cathode. Connectiony |83 establishes the equilibrium potential of the shield ring substantially equal to that of Aback plate 24 and tube control grid |80; and condenser |82 provides a low impedance connection to the tube cathode, by which the tube, acting as a cathode follower, drives the shield ring potential in direct accordance with the variable component of the grid voltage.

A result of such connection is Vthat the voltage across insulating bushing 36 of the back plate assembly is substantially zero, both as regards fixed and variable voltage components. Hence there is negligible leakage across that insulation, even if the material used is not selected primarily for its electrical properties. The full polarization voltage is applied across insulating bushing 34, but leakage across that bushing does not result in microphone noise, since shield ring 38 is isolated from the tube control grid, and is related to ground by a relatively low impedance. Therefore the material for both insulating bushings 34 and 36v can be -selected'with primary attention to its mechanical properties without appreciably affecting the electrical performance of the microphone. It is therefore feasible to use insulating bushings that have maximum rigidity and maximum stability against deformation with time, with changing atmosphericconditions and the like. That relative freedom of selection of insulating material has been a vital factor in the development of a practical condenser microphone of very small size and having remarkably high fidelity and signal strength.

Furthermore, the introduction of shield ring 38 does not increase the eifective stray capacitance of back plate 24, but rather the opposite. The eiect of stray capacitance between back plate and shield ring is substantially eliminated by the connection via condenser |82 to the tube cathode, the action of the cathode follower in that respect being functionally the same as its well-known action in reducing the effect of grid to cathode capacitance in the tube. The stray capacitance between the guard ring and ground is relatively ineffective because it shunts the relatively low output impedance of the preampliiler tube.

An alternative manner of connection, which operates on the same general principle but with less complete eiectiveness, involves a direct connection of shield ring 38 to tube cathode |60. That connection results, for example, if line |80 is disconnected from the junction of resistors |62 and |64 and is connected instead to the tube cathode, short circuiting condenser |82. There is then a slight difference in potential between the microphone back plate and shield ring, that: diierence being equal to the. control grid bias. of tube i4. Such connection leads to substan-y tially equivalent` capacity relationshipsas vthe preferredconnectiom but involves the possibility oi leakage currents across insulating bushing 36,

It will. be understood thatV the. embodiment defscribed and illustrated herein is: broadly illustra-f tive` of the invention,v andthat many. changes of structure and arrangementmay be. made without departing from the scope of the invention.. In particular, and as anexample only, the insulat-V ing support structure for'back plate 24,. shown as. a system` of concentric rings,`v or bushings, is not necessarily limited to that .type of geometrical form. One of the advantages of the invention isthat theinsulativerequirementsof that mounting are sc 'rmuch reduced that many mechanical arrangements not previously feasible for electrical reasons, are nowk available.

I claim:

1. A condenser microphone system comprising aV conductive `housing at eiective ground potential, a conductive microphone diaphragm electrically connected to the housing and supported therewithin for vibratory movement` in response to sound Waves entering the-housing, a conductive microphone plate` xedly mounted vwithin the housing in closely spaced parallelismy to the diaphragm and separated from ground by a relatively high impedance, A an insulating support interposed between the plate and the housingand comprising two mutually spaced elements of dielectric material separatedl from-each other by a conductive shield, an'electrical element separated from grou-nd by a relatively low impedance, circuitmeans for developing across the relatively high impedance between the microphone plate and ground a first voltage thatV comprises a rela tively large fixedl component and a relatively small variable component that varies in response to vibrations ofthe diaphragm, and for developin'g across the said relatively low impedane between the electrical element and ground ,a sec-f ond voltage comprising a ixed componentsubstantially equal to that 'of the` first voltage and a variable component which varies under control of; and in direct proportion to, the variable component of the rst voltage, and arelatively low impedance connection between the said elec.- trical element and the conductive shield.

` 2. 'Aconden'ser microphone system comprising a conductive housing at effective ground potential, a vconductive microphone diaphragm electrically connected to the housing and supported therewithin for vibratory movement in response to sound Waves entering the housing, a conductive microphone plate xedly mounted within the housing in closely spaced parallelism to the dia;- phragm and separated from ground by a rela.- tivelyA high impedance, an insulating support interposed between the plate and the housing and comprising two mutually spaced elements ofdi.- electric,l material separated from each other by aconductive shield, a vacuum tube including a plate, a control grid and a cathode, a load Vresistance. connected between the tube cathode and ground. a sourcey of voltage connected between the tube plate and ground,`an electrical connection between the microphone platey and the tubecOntrOl grid, and an alternating current connection between the shield and the tube cathode. l

3. A microphone system as defined in claim 2, and` in which the last said connection comprises a relatively large capacitance, and there is also a direct current connection between the shield and a point of the electrical system that. is at an average voltage from ground approximately equall to the average voltage from ground of the tube grid.

4. A microphone system as deiined in claim 3, and'v in which the said point of the electrical system is an intermediate point of the load resistance.

v PAUL S. VENEKLASEN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Y Date 1,780,671 Crippen Nov. 4, 1930 1,988,001 Flanders Jan. 15, 1935 2,113,347 Hoschke Apr. 5, 1938 2,321,370 Dubilier June 8, 1943 2,396,691 Galbreath Mar. 19, v1946 2,445,821 Brewer July 2 7, 1948 2,465,288 Sinett Mar. 22, 1949 

